Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing, or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bi-directional pulse transmission link between the pacemaker and the heart or may be used to deliver defibrillation shocks to the patient. A typical transvenous type pacing/sensing lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector terminal pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled or cabled conductor surrounded by an insulating tube or sheath typically couples the connector terminal pin at the proximal end and the electrode at the distal end.
The implantable cardiac stimulation leads with which the present invention is concerned may take the form of pacemaker leads capable of pacing and sensing in at least one chamber of the heart. Indeed, the present invention, may relate to a programmable dual chamber pacemaker wherein the basic configuration of the pacemaker, e.g. unipolar or bipolar, can be changed, including the grounding configuration and ground potentials used within the pacemaker.
Generally, a heart stimulator, commonly known as a “pacemaker” or “pacer”, uses one, two, or more flexible leads having one end connected to the pacer and the other end connected to electrodes placed in close proximity to the heart. These leads are used to stimulate or pace the heart. These leads are also used to sense the heart's electrical activity by sensing the heart's signals from their electrodes.
In order to properly pace or sense, the pacer has to be able to deliver a stimulating pulse to the heart or sense an electrical signal from the heart, and this requires that there be an electrical return path. If, within a given heart chamber, a unipolar lead is used—containing a single conductor—the return path is the conductive body tissue and fluids. The return path is connected to the pacer by connecting the pacer electrical common or ground to the pacer metal enclosure, typically referred to as the pacer case or housing. The case, in turn, makes contact with the body tissue and/or fluids. Then the current flows from the pacemaker through the lead's conductor, then through the lead's electrode, then through tissue, and finally to the pacer case.
An alternative solution to using a unipolar lead in a given heart chamber is to use a double lead/electrode in the heart chamber, known as a bipolar lead. In a typical bipolar lead, a second conductor coil or cable is spiraled over or positioned in a separate lumen and insulated from a first conductor along the length of the lead. At the distal end of the lead, one of the conductor cables or coils (inner) is connected to a first electrode, referred to as the “tip” electrode, and the second (outer) conductor coil is connected to a second electrode, referred to as a “ring” electrode. The ring electrode is generally situated about 10 to 20 mm from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is likely to be mostly in electrical contact with the blood. Because both body tissue and fluids are conductive, the ring electrode of a bipolar lead, in contact with the body fluids, serves as an electrical return for both pacing and sensing.
As indicated, pacing or sensing using the pacer case or enclosure as part of the electrical return path is known as unipolar pacing or sensing. Pacing or sensing using the lead ring electrode and an associated lead conductor as the electrical return path is known as bipolar pacing or sensing. There are numerous factors to consider when deciding whether unipolar or bipolar pacing and/or sensing should be used. Bipolar sensing is less prone to crosstalk, far field signals, and myopotential sensing than is unipolar sensing. Crosstalk generally refers to a pacer mistakenly sensing a heart activity in one heart chamber immediately after the other chamber is paced. Bipolar sensing reduces crosstalk resulting from a pacing stimulus in the opposite chamber. Bipolar pacing is also preferred as it minimizes the potential for pectoral or diaphragmatic stimulation.
Unipolar pacing and sensing offers the advantage, in general, of simpler circuitry within the pacemaker and a simpler, smaller diameter lead. In any event, cardiac pacing leads intended to be placed in the chambers of the heart, or nowadays, into the coronary venous system, are subjected to a series of tortuous bends that they must traverse. The leads must have the flexibility to follow these bends but have enough structural support to allow the leads to be pushed and twisted in order to navigate within these veins.
As earlier noted, the present invention has application to placing leads whose electrodes are either endocardial usage or epicardial usage. In the customary manner, a transvenous lead with an endocardial electrode provides an electrical pathway between the pacemaker, connected to the proximal end of the lead, and endocardial tissue in contact with the distal end of the lead. Endocardial tissue refers to a specific layer of tissue in the interior of the heart's chambers. In such a manner electrical pulses emitted by the pacemaker travel through the endocardial lead and stimulate the heart.
Transvenous leads with endocardial electrodes are often placed in contact with the endocardial tissue by passage through a venous access, such as the cephalic or subclavian veins or one of their tributaries. In such a manner transvenous leads offer as an advantage that their electrodes may be placed into contact with the heart without requiring major thoracic surgery. Rather, transvenous leads may be introduced into a vein and maneuvered therefrom so that their electrodes make contact with the endocardium of the heart.
A multi-step procedure is often used to introduce such leads within the venous system. Generally this procedure consists of inserting a hollow needle into a blood vessel, such as the subclavian vein. A wire guide is then passed through the needle into the interior portion of the vessel. The needle is then withdrawn and an introducer sheath and dilator assembly is then inserted over the wire guide into the vessel. The assembly is advanced into a suitable position within the vessel, i.e. so that the distal end is well within the vessel but the proximal end is outside the patient. Next, generally, the dilator and wire guide are both removed. It is also common to keep the wire guide in place, in case it is needed again. The introducer sheath is left in position and therefore offers direct access from outside the patient to the interior of the blood vessel. In such a fashion a transvenous lead can be easily passed into the vessel through the introducer sheath and ultimately be positioned within the heart. Finally the introducer sheath is removed from the body. With respect to pacemaker leads, however, which typically have a relatively bulky connector pin assembly at the proximal end, the introducer sheath is removed from the body by being split or slit apart. In such a manner the introducer sheath does not have to be large enough to be removed over the relatively bulky connector pin assembly at the proximal end of the lead.
An introducer sheath therefore, through its hollow lumen, provides access to the interior of a vessel. A transvenous lead introduced into the blood vessel may then moved along the blood vessel until properly positioned within the heart.
To provide such access an introducer sheath must be flexible. Specifically, flexibility permits the introducer sheath to bend and form to a curve compatible with the blood vessel. In such a manner the introducer sheath end is substantially parallel to the blood vessel and a lead which is introduced therethrough is properly oriented along the vessel interior. If the sheath did not conform to the vessel shape, a lead introduced would abut against the vessel wall, possibly injuring the patient and damaging the lead. One problem which may occur, however, due to the flexibility required of the introducer sheath is that the mid-portion of the sheath may form a kink.
Kinking within the introducer sheath may cause serious problems, especially with respect to pacemaker leads. Generally a kink within an introducer sheath is not detected until a lead is attempted to be introduced therethrough. At that time the lead, and in particular the delicate electrode at the distal end of the lead, strikes the kinked section and is blocked. Continual pushing on the lead may cause damage to the electrode as well as damage to the conductor and insulative sheath of the lead body. Because such damage may not be readily apparent, implantation of a damaged lead may result, in turn, creating the possibility of serious harm to the patient.
A further problem exists in pacemaker patients who have had multiple leads implanted over time. Scar tissue at the site of implantation has been found to create difficulties with past lead introduction systems. Specifically the relatively tough scar tissue hinders the introduction of a dilator and introducer sheath assembly. Many times, only through use of larger incisions than are otherwise desirable is such an assembly able to be inserted.
Others have proposed introducer systems that feature a sheath having a kink resistant section allowing the sheath to be bent in that region and still allow a lead to be introduced therethrough. The kink resistant section comprises a series of bellows or pleats. The bellows or pleats may be further arranged to form a screw about a portion of the sheath to thereby permit the sheath to be screwed into body tissue. The sheath preferably is constructed to readily split in a longitudinal direction and thus permits the system to be removed from the venous system without having to withdraw the sheath over an end of the pacemaker lead.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.